Well-Being Prevention & Cures

New CDC study compares cases and hospitalizations in people with previous infections and vaccinations

Story at a glance

  • The CDC released a study of case and hospitalization data for last year.
  • Patients were split into four groups based on vaccination status and whether they had a coronavirus infection before the study period.
  • The study authors emphasized that vaccination is still “the safest strategy.”

The Centers for Disease Control and Prevention (CDC) released a new study of data from May to Nov. 2021. In the report, they compare hospitalizations in people who were previously infected with the coronavirus and people who have been vaccinated. 

The researchers found that before the delta variant became the most common variant in June, case rates were higher among people who previously had the coronavirus compared to people who had been vaccinated. This trend changed by October, where they saw that it had flipped so that case rates were lower among people with previous infections than in people who were vaccinated. 

For the study, people were split into four categories: people who had a previous laboratory confirmed SARS-CoV-2 infection by March 1, 2021, people who had at least the first shot of a coronavirus vaccine, people who had a previous diagnosis of coronavirus and were fully vaccinated, and people who neither had a previous infection nor had been vaccinated. 

Before the delta variant became the main version of the coronavirus, the highest rates of infections were in unvaccinated people without a previous COVID-19 diagnosis, according to the report. Case rates were low in the other three groups and lowest in vaccinated people who did not have a prior infection. 

The team found that as the delta variant picked up, the case rates increased more rapidly in the vaccinated group with no previous diagnosis than compared to both the vaccinated and unvaccinated groups that had prior infections. 


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By the week of Oct. 3, the data suggest that case rates in vaccinated people without a prior infection remained many fold lower than in unvaccinated people without a prior infection. Among unvaccinated people, people with a prior infection had lower case rates than those without a prior infection. 

However, these data are tricky to interpret because of changing levels of immunity and the addition of new variants. The delta variant became the predominant variant in June and spread widely thereafter but how quickly rates increased could be due to a variety of reasons. 

In Nov. and Dec. 2021, the omicron variant emerged and has since become the dominant variant, which could further complicate interpretation of case data. This study does not include data for the omicron variant, so the results and conclusions may not be relevant to the current situation. 

The study authors write, “Although the epidemiology of COVID-19 might change as new variants emerge, vaccination remains the safest strategy for averting future SARS-CoV-2 infections, hospitalizations, long-term sequelae, and death. Primary vaccination, additional doses, and booster doses are recommended for all eligible persons.” 


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